Virtualization The XEN Approach Virtualization XEN: paravirtualization Computer
Virtualization The XEN Approach Virtualization XEN: paravirtualization Computer Laboratory References and Sources Paul Barham, et.al., Xen and the Art of Virtualization, Symposium on Operating Systems Principles 2003 (SOSP03), October 19-22, 2003, Bolton Landing, New York. Presentation by Ian Pratt available at http://www.cl.cam.ac.uk/netos/papers/2005-xen-may.ppt CS5204 Operating Systems 2 Virtualization Xen - Structure Employs paravirtualization
strategy Domain 0 Deals with machine architectures that cannot be virtualized Requires modifications to guest OS Allows optimizations has special access to control interface for platform management Has back-end device drivers Xen VMM entirely event driven
no internal threads Xen 3.0 Architecture CS5204 Operating Systems 3 Virtualization MMU Virtualizion : Shadow-Mode guest reads Virtual physical Guest OS guest writes Accessed & dirty bits Updates Virtual Machine VMM MMU CS5204 Operating Systems Hardware 4
Virtualization MMU Virtualization : Direct-Mode guest reads guest writes Virtual Machine Guest OS Xen VMM MMU CS5204 Operating Systems Hardware 5 Virtualization Writeable Page Tables : 1 write fault guest reads first guest write Virtual Machine Guest OS
page fault Xen VMM MMU CS5204 Operating Systems Hardware 7 Virtualization Writeable Page Tables : 2 - Unhook guest reads guest writes X Virtual Machine Guest OS Xen VMM MMU CS5204 Operating Systems Hardware 8 Virtualization
Writeable Page Tables : 3 - First Use guest reads guest writes X Virtual Machine Guest OS page fault Xen VMM MMU CS5204 Operating Systems Hardware 9 Virtualization Writeable Page Tables : 4 Re-hook guest reads guest writes Virtual Machine Guest OS
Driver isolated from VMM in its own domain (i.e., VM) Communication between domains via device channels Unified interfaces Common interface for group of similar devices Exposes raw device interface (e.g., for specialized devices like sound/video) Separate request/response from event notification I/O descriptor rings Used to communicate I/O requests and responses For bulk data transfer devices (DMA, network), buffer space allocated out of band by GuestOS Descriptor contains unique identifier to allow out of order processing
Multiple requests can be added before hypercall made to begin processing Event notification can be masked by GuestOS for its convenience CS5204 Operating Systems 11 Virtualization Device Channels Connects front end device drivers in GuestOS with native device driver Is an I/O descriptor ring Buffer page(s) allocated by GuestOS and granted to Xen Buffer page(s) is/are pinned to prevent page-out during I/O operation Pinning allows zero-copy data transfer CS5204 Operating Systems
12 Virtualization System Performance 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 L X V U L
X V U L X V U L X V U SPEC INT2000 (score) Linux build time (s)OSDB-OLTP (tup/s)SPEC WEB99 (score) Benchmark suite running on Linux (L), Xen (X), VMware Workstation (V), and UM Benchmark suites
Spec INT200: compute intensive workload Linux build time: extensive file I/O, scheduling, memory management OSBD-OLTP: transaction processing workload, extensive synchronous disk I/O Spec WEB99: web-like workload (file and network traffic) Fair comparison? CS5204 Operating Systems 13 Virtualization I/O Performance Systems
L: Linux IO-S: Xen using IO-Space access IDD: Xen using isolated device driver Benchmarks Linux build time: file I/O, scheduling, memory management PM: file system benchmark OSDB-OLTP: transaction processing workload, extensive synchronous disk I/O httperf: static document retrieval SpecWeb99: web-like workload (file and network traffic) CS5204 Operating Systems 14
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